Stretched surface recording disks (SD) generally consist of a rigid, circular support and a thin polymer film, having a recording layer, suitably attached to the periphery of the support. The development of SD has progressed over the years from a rather simple configuration described in U.S. Pat. No. 3,373,413 (Treseder) in which a film was stretched and clamped between two circular rings, to a dish-shaped support to which a stretched film was attached at the periphery as illustrated by U.S. Pat. No. 3,509,274. Subsequently, U.S. Pat. No. 3,537,083 introduced the concept of bonding the film at the center of the support in addition to the support periphery, and U.S. Pat. No. 3,599,226 described an SD which included two stretched film surfaces, one on either side of the support, which were attached at the periphery and near the center of the support.
SD are superior to floppy disks in that a pressure-pad is not required, and the recording surface has substantially the same overall dimensional stability as the relatively massive base to which it is attached. In addition, the SD provides a flat recording surface which may be deformed slightly to conform to a transducer head and irregularities in the surface of the head.
Although the patents described above have illustrated the general configuration and certain desirable features of SD, practical problems remain as impediments to the mass production and general acceptance of SD. These include the problems of track stability and stress relaxation of the stretched film.
Track dimensional stability is an important parameter in the design of recording media and drive systems. A change in track shape can cause a serious loss in signal-to-noise ratio or the loss of prerecorded data. It has been determined that circular data tracks originally recorded on SD became elliptical or distorted as the SD were exposed to variations in environmental conditions. This track movement was not completely reversible, i.e., the original circular configuration was not obtained when the SD were returned to original environmental conditions. Thus track density is limited by the dimensional instability or anisotropy of the tracks.
It has been found that the tension or stress present in the stretched recording sheet decreases over time even without changing environmental conditions. This is referred to as stress relaxation and can also distort the data tracks.